1. Field of the Invention
An aspect of the present invenion relates to a method of determining capability and quality of foundation piles and a method of designing foundation piles and an apparatus which is applied to the method for measuring characteristics of the ground. Another aspect of the present invention relates to a method of drilling a hole of a shape designed for foundation piles such as cast-in-situ piles to support a structure and an apparatus for the method.
2. Background Art
There are earth drill methods, overall casing methods, reverse circulation drill methods, etc. as cast-in-situ pile methods. In each method, a drilling machine drills a hole in a predetermined ground of a predetermined diameter by a predetermined depth. After the drilling machine is pulled out of the ground, a suspended tremie is put in the borehole to remove slime at the the bottom of the borehole. Then, a suspended rebar cage is moved down to the bottom of the borehole, and ready-mixed concrete is injected into the hole to fill the hole, while the tremie is being lifted up. Hardening of the concrete results in a cast-in-situ pile. Meanwhile, the foundation pile may be made with a prefabricated pile by filling the borehole with bottom consolidation cement slurry and inserting the prefabricated pile such as a concrete pile, instead of using the rebar cage. However, there have been some problems described below.
The supporting capability of the foundation piles, which may be the cast-in-situ pile or the prefabricated pile, for a structure is ordinarily determined in the following way.
As size, shape, etc. of the structure on the predetermined site are designed the vertical load, the lateral force by an earthquake or a wind, and the bending moment applied to the foundation pile are accordingly determined. A geological survey in the predetermined site is performed, foundation piles capable of enduring the above-mentioned forces are sougth, and the kind of the foundatio piles (the cast-in-situ pile or the prefabricated pile), the diameter of the pile, the length (depth) of the pile, the way of construction and the design bearing capacity are determined. According to the kind of structures constructed, the allowable settlement and the allowable lateral displacement, namely, the design deformation, after construction of the structure are also taken into consideration to determine the foundation pile and the way of construction.
However, the bearing capacity and deformation of the foundation piles considerably depend on the soil condition of the ground in which the foundation pile is to be placed, and they are not known until the foundation pile is placed in the predetermined ground and load is practically applied to the pile (i.e., a loading test). It takes many days to carry out the loading test, considering the entire term necessary for constructing the structure, and it is impossible to perform the loading test on every one of the piles, considering the term and the costs necessary for the construction. A cast-in-situ pile has in general a large bearing capacity, so that the loading test costs for a cast-in-situ pile become prohibitive.
Accordingly, the foundation pile is designed by an indirect method where its bearing capacity and deformation are determined from empiriical formulae which have been obtained by analyzing data of existing loading tests based upon geological survey data such as SPT-N values in the ground at the site.
However, with regard to application of the aforementioned indirect method, there is the disadvantage that when the cast-in-situ pile is made, namely, a hole for the pile is drilled by a drilling machine such as an earth drill, the wall of the borehole may crumble due to the vertical movement of the drilling machine, or the bearing capacity of the ground is reduced due to the decompaction and disturbance of the bottom of the borehole, so that the cast-in-situ pile can not be made as expected and specified in design.
The geological survey itself is restricted by time and cost and carried out only for a few parts of the vast site, where its soil condition may be heterogeneous, to be provided with lots of foundation piles. The bearing capacity of each of the many unsurveyed foundation piles is found by applying the above-mentioned soil condition data to the entire site, so that obtained values for the bearing capacity are inaccurate, and applying those values to practical construction is dangerous.
The empirical formula itself has the disadvantage explained hereinafter. In general, the loading test is performed in the condition that the foundation pile provided in the actual ground is loaded on its top with a yield load Py (the pile or the ground varies from an elasto-plastic state to a plastic state) or with an ultimate load Pu (the pile or the ground fails), as shown in FIG. 26. For the design bearing capacity, the deformation of the foundation pile is taken into consideration, and a smaller value, (1/2) Py or (1/3) Pu, is employed for practical provision of the foundation pile. In other words, the construction is uneconomically performed, taking an excessive safety factor.
The empirical formula is obtained by analyzing several loading test as stated above. FIG. 27 shows a graph in which the axis of abscissa represents the bearing capacity data of the pile obtained by the practical loading test and the axis of ordinate represents the bearing capacity of the pile calculated with empirical formulae based upon the geological survey data at the respective grounds sites of the loading tests. Data for a number of sites are plotted in the graph.
Empirical Formulae: EQU Pa=1/n(.alpha.Ap+.beta..sub.1 Af.sub.1 +.beta..sub.2 Af.sub.2) EQU Pu=(.alpha.Ap+.beta..sub.1 Af.sub.1 +.beta.Af.sub.2) EQU n=safety factor
In this case, if the bearing capacity of the pile obtained by the loading test corresponded to the bearing capacity of the pile calculated with the empirical formulae, the data should be plotted on a line inclined at an angle of 45.degree. (Pu) should in FIG. 27. However, since the empirical formulae themselves have been obtained by analyzing the aforementioned such data, few of the data platted points are on the line. A data group plotted above the Pu line shows that the bearing capacity of the pile calculated with the empirical formulae sometimes is larger than the bearing capacity of the pile obtained by the practical loading test, and if the design bearing capacity is determined with those calculations, it will apparently be extremely dangerous to employ them. On the other hand, a data group plotted below the Pu line proves that employing the design bearing capacity determined from empirical formulae is sometime too conservative and, hence, uneconomical. Adding a further safety factor for the latter cases is excessively conservative.
As has been described, after the design bearing capacity is determined for a single foundation pile with empirical formulae and the data such as the geological survey, allocation and disposition of the foundation piles to footings (i.e., foundation bases) for transferring the load of a structure to the foundation pile are carried out. The practical bearing capacity of each of the foundation piles is not known, and hence problems occur as follows:
Generally, the design bearing capacity of each of the foundation piles supporting a single structure is set to have a certain value (e.g. Pa=100 ton/pile). In allocating those foundation piles to the footings, the basic loads applied to the foundation bases in the footings are different from each other depending upon the shape of the structure and the variation in height of the structure. For example, assuming that the basic load in a footing F1 is 420 ton and the basic load in a footing F2 is 180 ton, allocations of the foundation piles to the footings are performed as follows: ##EQU1## Accordingly, the loads applied to a single foundation pile in the footings F1, F2 are different as follows: ##EQU2## As a result, the safety factors are also different between the footings F1 and F2. Thus, there is a difference in the loads which the piles support, and the depression and deformation after construction are different between the footings F1, F2. This result leads to an extremely uneconomical and dangerous setting of the design bearing capacity.
The execution of construction includes steps of (1) designing a structure, (2) determining the baseic load, the settlement and the deformation, (3) performing a geological survey and (4) determining the bearing capacity of a pile with empirical formulae based upon the survey data (the diameter and length of the pile), the number of the piles and the construction method. Originally, this way of construction where the unknown bearing capacity for each of the piles is detemined without practical experiments is very dangerous, and also uneconomical because a large safety factor must be employed to avoid danger.
As stated above, the practical bearing capacity of the cast-in-situ pile highly depends upon the soil condition of the ground to be provided with piles, the way of executing construction, etc. When the cast-in-situ pile is made, namely, when a hole is made by a drilling machine such as an earth drill, the wall of the borehole is loosened and crumbled due to the vertical movement of the drilling machine within the borehole, the bottom of the borehole is decompacted and disturbed, or the durability of the ground is reduced to result in the deposition of slime at the bottom of the borehole. These all cause the reduction of the bearing capacity of the pile, so that it is difficult to make the cast-in-situ pile as designed.
As mentioned above, the bearing capacity of the cast-in-situ pile depends upon the ground condition. The most part of the load of the structure is generally supported by the shaft bearing capacity of the pile under working load. However, there have been on attempts to press the borehold wall to compact the ground, to make a tapered borehole with regard to the depthwise direction, or to make an inversely tapered borehole to increase pull-out resistance, so as to enhance the shaft bearing capacity. In the case where the hole is tapered, the degree of taper is very small (e.g. 1 to 2%) though it depends on the soil condition of the ground. Although it is advantageous with respect to the shaft bearing capacity of the pile that the hole for the pile be tapered, there has been no way of accomplishing that.
Further, there may be employed a cast-in-situ pile having projections such as nodals on its peripheral surface so as to increase the shaft bearing capacity. However, it is difficult to make a hole having a required shape by simply using the drilling machine because the borehole wall crumbles. There are some ways of eliminating the decompaction and disturbance of the bottom of the borehole; a heavy deadweight is dropped down the hole, an inside sub-pile is put in and pressed, or a device for pressing the ground is inserted in the hole, so as to make the bottom of the hole compacted. However, there is also the disadvantage that the wall of the borehole crumbles, or the wall and the bottom of the borehole are decompacted during the operation of putting the pressing device into the borehole, so that the pile can not have enough end bearing capacity. There is another disadvantage that even when an inside sub-pile or a pressing device are inserted to press the bottom of the borehole, it is difficult to obtain enough reaction force to make the bottom compacted.
As has been described, the design bearing capacity of the foundation pile such as the cast-in-situ pile can merely be determined extremely uneconomically, inaccurately and dangerously, because there is no uniformity in respective practical bearing capacities of many piles for a structure due to the difference of the ground condition or the way of construction, or because there is no way of confirming the capability of the piles in supporting a structure. The present invention solves these problems and provides an apparatus for the solution. The present invention also provides a method of executing construction sufficiently suitable for using a bearing capacity characteristic of a cast-in-situ pile and an apparatus for the method.